This disclosure is related to the field of nuclear magnetic resonance (NMR) apparatus and methods. More specifically, the disclosure is related to NMR apparatus configured for measurement of surface and bulk NMR properties of very small liquid samples, for example, to detect the presence of certain substances in the very small liquid sample.
More particularly, the disclosure relates to methods and apparatus for using NMR for differentiation of fluid properties in the bulk of a fluid sample and in a layer of the fluid that interacts with a surface. In one aspect, methods and apparatus according to the disclosure relate to using NMR for rapid quantitative determination of cell conjugation. In another example aspect, methods and apparatus according to the disclosure relate to using NMR in toxicology as a rapid presumptive screen for certain classes of drugs. In yet another aspect, methods and apparatus according to the disclosure relate to using NMR in disease diagnosis to evaluate either the presence of an antigen or the presence of an antibody in a serum or other fluid sample.
The description herein and its background will be approached in the context of detecting the presence of an antigen in a sample. There is no intention to limit the generality of the present disclosure to the field of detecting the presence of an antigen in a sample.
Enzyme-linked immunosorbent assay (ELISA) is a test that uses antibodies and color change to identify a substance. In direct-ELISA a labeled primary antibody reacts directly with an antigen. Indirect-ELISA uses an unlabeled primary antibody in conjunction with a labeled secondary antibody. Since the labeled secondary antibody is directed against all antibodies of a given species, Indirect ELISA can be used with a wide variety of primary antibodies.
Antibody-sandwich ELISAs are a very useful type of immunosorbent assay for detecting antigens because they are frequently between 2 and 5 times more sensitive than those in which the antigen is directly bound to a solid phase. To detect the antigen, the wells of sample size microtiter plates (2 to 3 cubic centimeters) are coated with a specific (capture) antibody followed by incubation with test solutions containing an antigen. Unbound antigen is washed out and an antigen-specific antibody is conjugated to an enzyme (i.e., a developing reagent is added), followed by another incubation. Enzyme labeled antibody can be produced in a laboratory animal that produces passively adsorbed antibody, or from a different species immunized with the same antigen that is captured. Unbound conjugate is washed out and a substrate is added. After another incubation, the degree of substrate hydrolysis is measured. The amount of substrate hydrolyzed is proportional to the amount of antigen in the test solution.
NMR signals as used methods according to the present disclosure arise from the nuclei of hydrogen atoms in water molecules. Once generated, the magnitude of the NMR signal decays according to transverse (T2) and longitudinal (T1) relaxation properties of the water-containing material being analyzed. Spin-spin (T2) relaxation occurs when a given ensemble of oscillating hydrogen nuclear axis spins lose coherence. Loss of spin coherence is caused by macroscopic and microscopic fluctuations in the static magnetic field experienced by a freely diffusing nuclear axis spin. The former is commonly referred to as T2* relaxation and the latter as T2 relaxation. T2 relaxation contains information about the microscopic environment experienced by the hydrogen nuclei in the water-containing material. T2 relaxation can be measured independently from T2* by means of a specialized series of RF pulses and delays, called a CPMG (Can Purcell Meiboom Gill) pulse sequence. A CPMG pulse sequence removes the effects of macroscopic static magnetic field inhomogeneities to specifically measure the contribution from the microscopic environment, by creating a series of spin echoes. The relaxation time is significantly shorter for a molecule proximate a sample chamber surface or wall area, as compared to a molecule in the bulk volume. This is an effect of paramagnetic centers at a wall surface that causes the relaxation time to be faster.
T2 measurements can be carried out in real time during an analyte-induced response. T2 changes as a function of measurement time and the rate of T2 change can be correlated to a quantitative amount of analyte. The measured T2 values can be influenced by several assay, instrument, measurement, and processing parameters. For example, the measured T2 values may depend on the static magnetic field strength and homogeneity and the total spin echo measurement time. Additional parameters and variables may include valency and size of the analyte, and sample temperature. As a result, T2 values may increase or decrease with time.
Sample mixing and loading, as well as T2 measurements, can be completed in tens of seconds, making sample incubation the rate-limiting step for magnetic resonance switching (MRSw) measurements. Incubation times have may be as long as several hundreds of minutes. NMR measurement of spin-lattice (T1) relaxation and diffusion takes longer than T2 measurement, but can provide valuable information related to fluid-surface interaction.